Hitherto, there have been known simple driving type organic EL display devices wherein an organic EL element having an organic emitting layer sandwiched between electrodes is driven by means of an XY matrix electrode structure, and the devices are disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2-37385 and 3-233891.
In such a simple driving type organic EL luminous device, the so-called line sequential scanning is performed. Therefore, in the case that the number of scan lines is several hundred, the instantaneous brightness required is several hundred times observation brightness. Consequently, the following problems are caused.
(1) The driving voltage thereof is 2 to 3 times higher than that in the case of direct current constant voltage. For this reason, the luminescence efficiency lowers or the consumption power gets larger.
(2) Since the quantity of flowing electric current instantaneously becomes several hundred times larger, the organic emitting layer is liable to deteriorate.
(3) Since the electric current quantity is very large in the same manner as in the (2), a drop in voltage in wiring of the electrodes gets larger.
Therefore, in order to solve the problems which simple driving type organic EL luminous devices have, suggested is an active driving type organic EL luminous device having a thin film transistor, (which may be referred to as a TFT hereinafter), so as to drive organic EL elements.
Such active driving type organic EL luminous devices have characteristics that the driving voltage thereof is largely lowered, the luminescence efficiency is improved and further the consumption power can be reduced with compared to simple driving type organic EL luminous devices.
However, in the case that an active driving type organic EL luminous device having such advantageous effects has a compound having the light emitting property to which the triplet state contributes (hereinafter referred to as triplet-related compound), for example, an organic luminescent medium containing an iridium complex, a high luminescence brightness is obtained whereas a problem that the luminous life span is short is encountered. That is, in triplet-related compounds, the time when the molecules thereof are excited and relaxed is longer in the case of compounds having the light property related to the singlet state. Hence, electric charge is stored inside, so that the balance between holes and electrons is liable to be destroyed. Thus, according to Japan Journal of Applied Physics, Vol. 38, pp. L1502-L1504 (1999), in the case of an organic EL luminous device having an organic luminescent medium containing an iridium complex, the luminescence efficiency thereof is a value of 40 lumens/W or more under the condition that the luminescence brightness is 500 cd/cm2 but the half life thereof is a short time of 200 hours or less.
Thus, the present inventors made eager investigation on the above-mentioned problems. As a result, it has been found out that an organic EL display device which consumes a low electric power and is drivable for a long time can be provided by driving the device through an appropriately-set driving circuit even if a combination of a host compound with a triplet-related luminous compound is used; accordingly, the display device can be applied to the field of flat panel displays and others.
That is, an object of the present invention is to provide an organic EL display device which consumes a low electric power and gives a less drop in luminescence brightness even when the device is driven for a long time.
Another object of the present invention is to provide a driving method capable of driving such an organic EL display device at a low consumption power for a long time.